I live in a 3 bed det house and 4 years ago had a new Worcester boch boiler fitted and the majority of the radiators changed for new modern ones and all was fine. Recently have changed 3 of the old remaining rads upstairs for newer ones and since then the problems started. Upstairs is great however downstairs now it now takes about 90 mins before all the rads are on. When they are on there are 3 in particular where at anytime only 2 out of 3 come on and the other doesn’t. It’s not always the same one.Engineer has been back and checked the system and claims there is nothing wrong. Advice please.
It's probably not air as that would rise to the top of your system, but you should make sure your rads are bled anyway as you never know. Also make sure your system is pressurized in the green arc, usually 1.2 bar. Check the simple things if you haven't already, like make sure the valves the rads are fully open.
You need to determine if this is a balancing issue or something more sinister. First turn off your system and let it get cold, then turn off the all but the problem rads. Then, turn your system on again, your downstairs rads should heat up quick. If they don't then there's probably an issue, perhaps there's some gunk restricting one of the pipes or there's another problem that needs attention.
If the rads heat up quickly the most likely issue is that the radiators aren't balanced. If you had old radiators in part of the house then they most likely had less flow than the new ones, so your other radiators were probably partially restricted to force water to them. Now you have nice new ones and they're taking all the hot water.
This is called a lockshield valve.
Its job is to close one of the ends of your radiator to balance the system. The valve on the other side is your more typical adjustment, probably a thermostatic valve. You may have a lockshield with a knob to turn (happy happy joy joy), or one of these pictured, in which case you have to pull off the plastic cover and use a flat head screwdriver (pain in the ass). If you have these be careful not to open them too much or the screw might come out and water will leak out.
First, go to the radiators that heat slowly and open their lockshield valves fully, then go to the radiators that heat up really fast and close theirs two full turns. You may or may not see an improvement at this point. Keep restricting the flow where the rads heat up quickly, half a turn at a time until you get the desired result. This can be a lot, I've had radiators where the lockshield was open 1/4 turn only.